Method for catalyzing a fuel cell electrode and an electrode so produced

ABSTRACT

A porous conducting particle, hydrophobic bonded, substrate supported electrode is prewetted with the electrolyte. A D.C. voltage is applied to the electrode to assist in the prewetting with the electrolyte. A soluble catalyst-containing material is then introduced into the electrode structure and the catalyst deposited within the electrode. By appropriate selection of the porous conducting particles and the catalyst-applying techniques, precise control of the location of the catalyst can be obtained. If graphite materials are used as the conducting particles, a catalyst-containing salt is allowed to dissolve in the electrolyte in the prewetted electrode, and the catalyst-containing material is reduced to the metal. If the reduction is done by reaction with a reducing gas such as hydrogen, the catalyst will be deposited only in those regions of the electrode at which there is an electrolyte-reactant gas interface which is in electrical-conducting relationship with the substrate. Alternatively, extremely precise amounts of catalyst can be deposited within the electrode structure by use of a solution of a compound of the catalyst whose wettability with the hydrophobic material varies as the solution evaporates. By this technique almost 100% of the catalyst can be deposited within the electrode structure on the hydrophilic region, with virtually no losses in the hydrophobic material.

This is a division, of application Ser. No. 533,918, filed Dec. 18,1974, now U.S. Pat. No. 3,752,197.

BACKGROUND OF THE INVENTION

1. Field of the Invention -- This invention relates to electrochemicalcells and more particularly, to a method of catalyzing electrochemicalcell electrodes and the electrodes so produced

2. Description of the Prior Art -- A well-known and important type ofelectrochemical cell is a fuel cell which reacts a fuel and an oxidantat a pair of electrodes to make electricity. Low temperature fuel cellsrequire catalysts in each of the electrodes to promote the reaction ofthe fuel and the oxidant. But, the electrochemical reaction of each ofthe reactant gases takes place in the presence of the catalyst only inthose regions of an electrode in which the electrolyte and the reactiongas establish an interface and the electricity produced can be takenaway. If there is catalyst at other places in the fuel cell electrode,or if catalyst is lost in processing, that catalyst is wasted. Catalyststypically used in fuel cells are expensive noble metals and therefore itis desirable to reduce the waste as much as possible and still have goodfuel cell performance; that is, efficient utilization of the catalyst isessential. To achieve an electrode in an electrochemical cell that canprovide a high current density and maintain a high voltage, it isnecessary to have a large electrolyte/reactant gas interface area. It isknown in the prior art that use of small and distinct hydrophobic areasthrough which the reactant gas can pass and hydrophilic areas in whichthe electrolyte can be present, allows for large interfaces. In one typeof electrode having these characteristics which has found wideacceptance, catalyzed agglomerates of porous carbon particles are boundtogether with polytetraflurorethylene (PTFE) to establish thehydrophobic and hydrophilic areas. One known method for catalyzing suchelectrodes applies a catalyst to the carbon particles before the carbonparticles are bound together with the PTFE and put onto a currentcollector support to form an electrode: this is known aspre-catalyzation. The pre-catalyzation method deposits catalyst on allof the carbon particles that are to be used in the electrode and, as aresult, some of the catalyst is wasted because some of it is placedwhere there may be no electrolyte/reactant gas interface or where thereis no electrical path out of the cell. The precatalytic techiniques alsoinvolve losses in the original treatment of the carbon as well as in thehandling of the catalyzed carbon during fabrication of the electrode.

In addition, it has sometimes been observed that precatalyzed electrodesdo not function as efficiently as electrodes which are fabricatedaccording to "post-catalyzation techniques." A post-catalyzationtechnique is one in which the catalyst is deposited in the electrodestructure after the electrode structure has been formed. Althoughresulting in superior performing electrodes all post-catalyzationtechniques hitherto employed have been extremely difficult processes tocontrol. This results in a substantial amount of the catalyst beingdeposited in areas where it is not desired. According to this invention,we have found a method by which post-catalyzation of conductingparticle, hydrophobic bonded substrate supported electrodes can beobtained by simple and extremely controllable techniques.

It is, accordingly, an object of this invention to provide anefficiently catalyzed electrochemical cell electrode.

Another object of the present invention is to provide a method forpost-catalyzing an electrochemical cell electrode.

It is another object of this invention to provide a method forpre-wetting, with an electrolyte, an electrochemical cell electrode.

It is another object of this invention to provide methods forselectively depositing a catalyst in a prewetted electrochemical cellelectrode.

These and other objects of the invention will be readily apparent fromthe following description with reference to the accompanying drawingswherein:

FIG. 1 is a schematic section through a conducting particlehydrophobically bonded substrate supported electrochemical cellelectrode and

FIG. 2 is a cross-section view of apparatus for carrying out theprewetting step of this invention.

DESCRIPTION OF THE INVENTION

Referring now to the drawings, FIG. 1 represents a schematiccross-sectional view through a typical, porous conducting particle,hydrophobic bonded substrate supported electrode. Such an electrodeconsists of a substrate 1 formed from a gas-permeablecorrosion-resistant electrical conductor, which in the conventionalphosphoric acid electrolyte fuel cell is a thin paper formed fromgraphite or carbon fibers, hereinafter referred to as carbon paper. Thecarbon paper substrate 1 may also be coated with a thin layer ofhydrophobic material such as PTFE to render it resistant to floodingwith the electrolyte which would destroy the gas permeability of thesubstrate 1 but this coating of hydrophobic material must besufficiently thin as not to interfere with the electrical conductingcapacity of the substrate 1. The body of the electrode consists ofporous electrically conductive particles 2 having catalyst deposited onthe surface. It should be recognized that the particles 2 are inactuality agglomerates of smaller porous particles. However, thedetailed structure of these particles is not important to anunderstanding of the invention and has been omitted from the drawing forclarity. The particles 2 are bonded together by hydrophobiccorrosion-resistant microspheres 3. It will be understood that themicrospheres 3 fill substantially the entire volume between theparticles 2 between the electrolyte matrix 4 and the substrate 1,whereas in the drawing they are only shown in certain locations, beingomitted from the remainder of the drawing for purposes of clarity. Theconducting particles 2 formed of a corrosion-resistant electricalconductor must be wetted by the electrolyte which is stored in matrix 4.Suitable materials for the particles being carbon black (e.g., CabotVulcan XC 72) or graphitized carbon black. The preferred hydrophobicmaterial is polytetrafluoroethylene (PTFE). The face of the electrodeopposite the substrate 1 abuts against a fibrous matrix 4 which isfilled with the fluid electrolyte which, in the case of a phosphoricacid fuel cell would be concentrated phosphoric acid. In operation ofsuch a fuel cell, the fuel or oxidant gas would be caused to flow pastthe substrate 1, diffusing into this electrode through the spacesmaintained by the hydrophobic microspheres 3; the hydrophobic nature ofthe microsphere preventing the electrolyte from entering into thesegas-flow channels. The electrolyte from the matrix 4 flows through thepores and across the surface of the conducting particles 2. At thoseplaces on the surface of the conducting particles 2 where there is aninterface between the electrolyte covering the catalyst and the reactantgas, a chemical reaction (either oxidation or reduction depending uponwhether the electrode is an anode or a cathode, respectively) occurs.The electron transfer with the external circuit occurs across currentconducting paths between the conducting particles 2 and the substrate 1and the ion transfer occurs by diffusion between the electrolyte matrix4 and the electrolyte on the particles 2.

As can be seen from FIG. 1, certain of the conducting particles whichare designated as 2a are surrounded by the hydrophobic material 3 and assuch are insulated from the current collecting substrate 1. Since theseparticles are electrically insulated from the substrate, no electrontransfer to or from these surfaces can occur. Thus, no chemical reactioncan take place on the particle and any catalyst which is deposited onthese particles will be wasted. According to one embodiment of ourinvention, this aspect of catalyst waste can be avoided. According toother embodiments of our invention, deposition of catalyst on particles2 is permitted to occur. However, the overall post-catalyzation processis carried out in a simple and economical manner without externalwasting of any catalyst or the need to recover catalyst from theprocessing step.

According to our invention, the electrode is prewetted with theelectrolyte and then a soluble catalyst-containing material is caused toenter into solution with the electrolyte on the surfaces of theparticles 2 which have been wet by the electrolyte. A chemical reactionis then caused to occur which deposits the catalyst on the surface ofthese particles. Thus, in order to successfully accomplish thepost-catalyzing process of our invention, it is necessary to solve twoproblems; (1) how to prewet the electrode with the electrolyte and (2)how to deposit the catalyst on the surfaces of the prewetted particles2.

In order to prewet the electrode, it is necessary to cause theelectrolyte to completely cover and penetrate through the porousconducting particles 2. When the conducting particles 2 were made ofcarbon black, it was possible to obtain prewetting by simple wickingaction which occurs when the electrode is placed in contact with theelectrolyte matrix. Since carbon black is reasonably wettable by theelectrolyte, electrolyte will, with time, penetrate through the poresand cover the surface of the particles 2, but a substantial period oftime is required to obtain complete wetting. When the conductingparticles 2 are made of graphitized carbon black, wicking does not occuror occurs with extreme slowness because the surface is not readilywettable by the electrolyte. In such instances we have found that it isnecessary to prewet the electrode by means of an anodic potentialapplied to the electrode which renders the surface of the graphitizedparticles wettable with electrolyte, probably by the formation of oxidesof carbon on the surface at the interface between the electrolyte andthe carbon. When used with graphitized carbon black, this prewettingapproach causes electrolyte to wet only those particles which are inelectrical-conducting relationship between the electrolyte matrix andthe conducting substrate. Thus, particles such as 2a and 2b will not bewet with electrolyte since they are not in current carrying relationshipto the substrate across which the potential is applied. This process ofanodically wetting the electrode can also be utilized with wettableconducting particles such as those formed of carbon black and in thisinstance it substantially reduces the time required to completeprewetting of the electrode. In this instance, however, since theconducting particles are wettable, particle 2b would be wet by theelectrolyte through simple wicking and particle 2a could conceivablybecome wet if a liquid passage occurred in the electrode structure as aresult of some imperfection in the way it was manufactured which wouldpermit electrolyte to reach the surface of this particle. The voltageand current required to cause the prewetting is selectable withinrelatively wide limits, a minimum voltage potential of approximately 0.9volts above the hydrogen potential being required to cause thegeneration of some oxide at the interface between electrolyte and thecarbon. In practice, a voltage in the range of 0.9 to 1.6 volts at acurrent density of 1 to 2 milliamperes/cm² has been found to producesatisfactory results within a reasonable period of time. Obviously, thelower the current density, the longer the reaction time, and operatingbelow the minimum potential of approximately 0.9 volts will not producethe necessary oxide formation. Operating above 1.6 volts has not beenfound to materially enhance the process.

Referring to FIG. 2, the apparatus by which the electrode can beprewetted is illustrated. An uncatalyzed electrode 10 corresponding instructure to that shown in FIG. 1 is placed in a container 40 filledwith the electrolyte which, in this case, is concentrated phosphoricacid 42, preferably with the conducting substrate 1 above the surface ofthe electrolyte 42 with the layer of conducting particles andhydrophobic particles immersed in the electrolyte. This arrangement isdesirable in order to prevent the substrate 1 from becoming wet with theelectrolyte which could destroy its hydrophobic character and obstructgas flow in the subsequent operation of the fuel cell. The electrode 10which is to be wetted by the electrolyte 42 rests upon a nonconductingporous matrix 46 such as tissue quartz which is used to electricallyinsulate the electrode from its counter-electrode. This matrix 46 inturn rests on a metal screen counter-electrode 48 which in thisembodiment is the negative electrode and which is, in turn, supported ona porous spacer 49 which is used to permit the evolution of any gasgenerated at the electrode 48. The entire assembly rests on supports 50which maintain the assembly off the bottom of container 40 and at theproper elevation within container 40. The substrate 1 of the electrode10 to be wetted is connected via rheostat 56 to the positive terminal ofa power source 54 and the counterelectrode is connected by a switch 52to the negative plate of power source 54. When switch 52 is closed and aproper appropriate adjustment made of rheostat 56, a positive voltage ofbetween 0.9 and 1.6 volts can be applied across the electrode 10 whichresults in the evolution of some hydrogen gas at negative electrode 48and presumably the production of some carbon oxide at the interfacesbetween the electrolyte and the current carrying conducting particles 2of electrode 10. Referring now to FIG. 1, it will be seen that sincegraphitized particles 2a and 2b are not in electrical-conductingrelationship with substrate 1, no wetting of these particles will occur,whereas particles 2 will be wet with the electrolyte. It has been foundthat electrode 10 is filled with the electrolyte in less than tenminutes and this time period for any particular configuration ofelectrode is readily determined experimentally by weighing the electrodeto ascertain the amount of acid that has enetered into the electrode andby performance data on the resultant electrode. Similar results are alsoobtained when the conducting particles 2 are formed from a wettablematerial such as carbon black. In this case, the particles would havebeen wettable with the electrolyte purely by wick action. However, as aresult of the anodic potential, the treatment time is reduced from overan hour to a few minutes. In this case, however, particles such as 2bwill become wet with electrolyte as a result purely of the wicking andparticles such as 2a may become wet with the electrolyte in the event ofsome imperfection in the electrode matrix which would permit a liquidpassage from the electrolyte bath to the particles 2a.

Having thoroughly prewet the electrode with the electrolyte via thetechnique described above, it now becomes necessary to deposit asuitable catalyst on the surface of the conducting particles 2. Variousmaterials are known to the art to be usuable as catalysts; however, themost highly efficient known to date is platinum. This invention will bedescribed with respect thereto, it being recognized that correspondingtreatment of other soluble compounds of catalytically active metals canbe used according to our invention. According to one embodiment of ourinvention, a solution of soluble, catalyst-containing compound is placedin contact with the prewet electrode and the soluble catalyst containingcompound is permitted to diffuse into the electrolyte contained onconducting particles 2. This, referring now to FIG. 1, a porousabsorptive mat similar to blotting paper and resistant to acids isimpregnated with a solution of chloroplatinic acid and placed in contactwith the electrode in the location shown as 4 on FIG. 1. Thechloroplatinic acid contained on this mat will diffuse into theelectrolyte contained on particles 2 and after a period of time, whichis dependent upon the concentration of chlorolplatinic acid in the mat 4and the configruation of the electrode itself, an amount ofchloroplatinic acid will have diffused substantially uniformly acrossall of the conducting particles 2 which have been coated by theelectolyte by the process described above. The electrode is then removedand the surface of the electrode blotted to remove the surface film ofexcess catalyst. The soluble catalyst containing material is thenchemically modified to precipitate metallic catalyst. This can beaccomplished, for example, by a chemical oxidation-reduction reaction tocause metallic platinum to be deposited on the surface of the conductingparticles 2. Such a reaction can be conveniently conducted by passinghydrogen gas (preferably 120°F to 400°F) through the supportingsubstrate 1 causing chemical reduction of the chloroplatinic acid todeposit platinum on the surface of conducting particles 2. If theelectrode structure has been formed from graphitized carbon black,anodically wetted as described above, particles 2b and 2a will notcontain any electrolyte and will not have any platinum depositedthereon. In this embodiment of the invention, the platinum is depositedwithin the electrode structure only in those areas where a chemicalreaction with appropriate electron transfer can occur, and thus theplatinum within the fuel cell electrode is efficiently utilized. Whencarbon black is used as the conducting particles, for example, somecatalyst deposition will occur on particles such as 2b.

since the electrolyte into which the catalyst containing compounddiffuses is located not only at the surface of particles 2 but alsothroughout the porous body of these particles; in some cases theelectrode can be treated to prevent the catalyst from enetering into theelctrolyte in the pores. Such a treatment would result in the catalystbeing deposited on the surface of the particles rather than within thepores. One approach taken was to freeze the electrode therebysubstantially reducing the rate at which the catalyst containingmaterial could enter the pores but without effecting the rate at whichthe solution would wick into the electrode structure.

When highly concentrated phosphoric acid is the electrolyte, a separatefreezing step is not necessary because the phosphoric acid is a solid atroom temperature in the presence of large numbers of nucleation sitessuch as the carbon itself. In other systems a freezing step or someother pore blocking technique could prove desirable and the use of sucha step is contemplated by this invention when needed.

The diffusion procedure, although desirable from the point of view ofmaximum catalyst utilization, does requrie the recovery of thecatalyst-containing material from the matrix or mat from which thecatalyst-containing material was allowed to diffuse into the electrodeand from the blotter used to remove the surface film. This requires anadditional processing step. According to another embodiment of thisinvention, there is no need for any recovery step since all of thecatalyst-containing material applied to the electrode is ultimatelydeposited within the electrode on conducting particles 2. According tothis embodiment some catalyst is deposited on particles such as 2a and2b, but this embodiment has the advantage of obtaining extremely precisecontrol of the actual amount of catalyst deposited within the electrode.According to this embodiment of the invention, we have found thatcatalyst-containing material can be caused to dissolve in a mixture oftwo soluble liquids, the wetting characteristic of which will vary withthe concentration of the two liquids. Thus, by appropriate selection ofthe catalyst-containing compound and the liquids, it is possible toproduce a solution of the catalyst-containing material which, during theinitial phase of the process, is capable of wetting the hydrophobicportions 3 of the electrode but which upon evaporation will becomenon-wettable with respect to these hydrophobic portions. The result isthat as the solution evaporates, the solution will be driven out of thehydrophobic regions and into the hydrophilic regions where thecatalyst-containing solution will contact the electrolyte on thesurfaces of the conducting particles 2. The catalyst-containing materialwill enter into solution in the electrolyte on particles 2 and can bedeposited in the same manner as described above by reduction with hothydrogen.

In order to obtain a solution which will change its wettingcharacteristics upon evaporation, it is necessary to have a polarcomponent and a relatively non-polar component as the two liquids usedto form the solution. The original mixture must have a non-polarcomponent which is more volatile than the polar component, or aconcentration on that side of an azeotrope which will cause theconcentration of the polar constituent of the solution to increase asevaporation occurs. Naturally, the catalyst-containing compound must besoluble in the polar component and in the solution of the mixtureoriginally chosen. While various materials can be used, we have foundthat water is the preferred polar component and the lower alcohols arethe preferred non-polar components. These materials are miscible in allproportions and the vapor pressures are such that evaporation can becaused to occur at moderate temperatures. While other materials meetingthe criteria described above, which are readily determinable by workersskilled in the art, can be used, the following description will bedirected to the use of water and alcohol.

When certain liquids are dissolved in each other, there will be acertain composition, which is unique for each set of materials, at whichthe composition of the vapor phase formed by evaportion of the mixtureis identical in composition to that of the liquid phase. This particularcomposition is known as the azeotropic composition. Any solution ofliquids at the azeotropic composition, therefore, cannot be separated bydistillation techniques since the composition of the liquid and vaporphases is identical. As an example, assume that the low-boilingazeotropic mixture of liquids A and B is 70% A and 30% B. If a liquidmixture is prepared having a smaller proportion of component A, say60/40, the vapor phase formed upon evaporation of this liquid mixturewill have a higher proportion of component A than the liquid phase.Thus, if the vapor phase is removed and evaporation continued, theconcentration of the liquid phase will continuously change andeventually the liquid phase will consist primarily of component B.Similarly, on the other side of a low-boiling azeotrope, if the initialcomposition was 80% A and 20% B, the vapor phase will have a higherproportion of component B than the liquid phase and if the vapor phaseis continuously removed with evaporation continuing, eventually theliquid phase will consist primarily of component A. According to thisinvention, we have found that solutions of polar and non-polar liquidssuch as alcohols and water become capable of wetting hydrophobicmaterials such as PTFE at composition ranges well below the azeotropiccomposition and on that size of the azeotrope at which evaporation willcause the concentration of the polar liquid in the liquid phase toincrease. When the concentration of the polar component reaches apredetermined level, the solution will cease to wet the hydrophobicparticles 3 and can be observed to physically withdraw from thesehydrophobic regions. However, particles such as 2a, if formed of a midlyhydrophobic material such as graphitized carbon black, will remainwetted with the solution since once wetted the concentration change isnot sufficient to restore the hydrophobic character. Thus, according tothis embodiment of the invention, the pre-wet electrodes manufactured asdescribed above can be thoroughly flooded in both the hydrophophilic andthe hydrophobic areas, by a solution of a catalyst-containing materialin the mixed solvents at a concentration at which the solution will wetthe hydrophobic regions. When the electrode is subject to heat or vacuumthe solution will begin to evaporate, becoming more concentrated in thepolar material and, when the concentration is reached at which theliquid solution is no longer capable of wetting the PTFE, the solutionwill withdraw itself from the hydrophobic regions 3 and mix with theelectrolyte within the porous particles 2. Since the catalyst-containingmaterial is in solution in this liquid, virtually all of the catalystsolution is ultimately located in the electrolyte within the conductingparticles 2. The catalyst-containing material can then be chemicallyreacted as described above in order to deposit the metallic platinum. Aspreviously noted, pore "blocking" techniques can be used if desired toprevent the catalyst from entering into the electrolyte in the pores ofparticles 2.

As an example of a specific preferred system which is usable accordingto our invention, a solution of isopropyl alcohol in water begins toexhibit wettable characteristics with respect to PTFE when it containsapproximately 30 volume percent alcohol. The azeotropic composition,however, is approximately 90 volume percent alcohol, and accordingly,any solution of water and alcohol in which the percentage of alcohol isgreater than approximately 30 volume percent and less than approximately80% will not only be capable of wetting the PTFE but will also becapable of becoming more aqueous as it is subjected to heat. When thepercentage of water in the solution exceeds 70%, the solution willbecome incapable of wetting the PTFE and will withdraw from thehydrophobic regions of the electrode into the hydrophilic regions andinto solution in the electrolyte on particles 2. It should be noted,however, that even if the electrode has been prewetted by the techniquedescribed above in which particles such as 2a and 2b are not wetted bythe electrolyte, some deposition of catalyst on these particles willoccur if the catalyzation technique just described in employed. This isbecause the graphitized particles 2a and 2b, although not hydrophilicwith water, are wettable by the alcoholic solution and once they havebeen so wetted, increasing the concentration of the water does notresult in the same change in wetting characteristics as is observed withthe PTFE. Thus, this process does result in the deposition of somecatalyst in areas where chemical reactions in the electrochemical cellcannot occur. However, this slight disadvantage is substantiallyovercome by the obvious processing advantages obtained when all of thecatalyst-containing solution can be caused to enter into theelectrochemical cell electrode in a highly controllable manner with noreprocessing and waste associated therewith.

The above describes the applicants' broad invention in a manner suchthat any person skilled in the art can practice the same with at mostroutine experimentation to determine the specific operating parametersfor any particular combination of materials. The following examplesrepresent preferred embodiments of the invention and are to beconsidered as supplementing the above disclosure rather than limitingthe same.

EXAMPLE 1

A gaseous diffusion electrode comprising a conductive carbon papersubstrate carrying an electrode body of porous graphitized carbon blackagglomerates (0.5 - 5μ), bonded together and to the carbon papersubstrate by PTFE particles (0.2μ) was weighed and placed in theapparatus of FIG. 2 with concentrated phosphoric acid (98%) at 70°C andsubjected to an anodic potential of approximately 1.2 volts at a currentflow of approximately 2 milliamperes per square centimeter for a periodof 15 minutes. Upon removal from the prewetting apparatus, the electrodewas blotted to remove surface acid and upon visual observation theelectrode appeared dry. The electrode was weighed and then treated asdescribed above for another 5 minutes, blotted dry and weighed again. Noweight gain was observed, indicating that total prewetting had occurredby the initial treatment. Without anodic treatment, the time requiredfor complete filling varies from 50-500 hours depending on the sample.

EXAMPLE 2

An electrode structure similar to that of Example 1 but using carbonblack (non-graphitized) as the conducting particle was treated accordingto the same process and similar results obtained. The electrodestructure appeared dry; however, it was thoroughly wetted by theelectrolyte. The anodic preparation obtained total impregnation in 5minutes whereas without the anodic treatment prewetting by wick actionrequired 0.5 to 3.0 hours for total prewetting depending on the sample.

EXAMPLE 3

A quartz mat was disposed in a tray and thoroughly impregnated with asolution of 25 mg pt/ml soltuion prepared by mixing chloroplatinic acidin 96% phosphoric acid. The prewet electrode prepared according toExample 1 with the carbon paper substrate on top was laid upon thequartz mat. The rate of diffusion of the chloroplatinic acid into thephosphoric acid electrolyte on the conducting particles is a function ofthe concentration of the chloroplatinic acid and the temperature of thetreatment. At 70°C, the impregnation of the electrode with the catalystwas continued for 5 minutes. The electrode was then removed, the surfaceof the electrode blotted and the blotter saved for later reprocessing torecover residual platinum. The electrode was then treated with hydrogengas at 150°C which was passed through the carbon paper substrate intoand through the electrode. Chemical reduction of the chloroplatinic acidto metallic platinum occurred and the concentration of platinum in thefinished electrode was 0.3 mg/cm². As prepared the platinum presentshould be located excusively on conducting particles which were inelectrical-conducting relationship with the substrate and thus capableof participating in the chemical reaction in the electrochemical cell.The electrode was capable of use in an electrochemical cell without theinitial start-up period required when the electrochemical cell electrodeis not prewetted. Thus, while the electrode appeared dry, it was in factready for immediate use rather than having to be impregnated with theelectrolyte after assembly into an operating cell.

EXAMPLE 4

Two milliliters of a solution of chloroplatinic acid in a mixture of 80volume percent isopropyl alcohol and 20 % water was placed in a trayhaving the following dimensions -- 3 inches by 3 inches. A prewetelectrode fabricated according to Example 1 and having dimensionscorresponding to that of the tray was laid on top of the solution ofchoroplatinic acid with the carbon paper substrate facing upwards. Thesolution was substantially completely absorbed in the electrode and uponremoval from the tray no noticeable solution remained in the tray. Theelectrode was heated at 110°C for 0.25 hour to evaporate the isopropylalcohol/water mixture. As the concentration of the solution approaches30% isopropyl alcohol, the solution becomes non-wettable with respect tothe hydrophobic PTFE portion of the electrode and the solution shouldretreat therefrom into the remainder of the electrode structure. Aftersubstantially all of the isopropyl alcohol/water solution had beenevaporated, the chloroplatinic acid was subject to gaseous reduction inthe same manner as Example 3. The finished electrode was placed inoperation in a fuel cell and produced cell voltage of 0.620 volt at acurrent density of 200 ma/cm² with hydrogen gas fuel and air oxidant ata temperature of approximately 190°C.

While this invention has been disclosed with respect to certain specificembodiments thereof, these embodiments are considered to be illustrativeand not limiting of the invention. The invention is usable withmaterials other than those specifically disclosed herein. It isanticipated that workers skilled in the art will, in fact, utilize othermaterials, and such use in contemplated within the scope of thisinvention so long as the general criteria duly set forth above are metwith respect to the selection of these various materials. For example,while this invention has been described with respect to an electrode fora phosphoric acid fuel cell, it is equally usable with basic KOHelectrolyte fuel cells as well as sulphuric acid fuel cells. Also,materials other than PTFE, graphite, carbon and platinum can be used.Suitable porous conducting particles for electrodes other than carbonand graphite are, for example, boron carbide, tantalum, and nickel,depending on the electrolyte environment. Suitable hydrophobic materialsinclude fluorinated ethylene-propylene and polystyrene, for example, andsuitable conducting substrates can be made from metallic screens orresin-bonded carbon plaques depending on the environment. Also, withrespect to the catalyst impregnation techinque, materials other thanalcohol and water can be used. Water, because of its availability anddesirable boiling point is, of course, the preferred polar material andsimilar comments apply also to the alcohol. However, a large number ofother organic liquids can obviously also be used. The specific operatingparameters for any particular combination of materials is believedreadily determinable by workers skilled in the art. Further thetechnique of increasing the wettability of the conducting particles bymeans of a D.C. potential can be used alone or in connection with otherprocesses such as the wetting of pre-catalyzed electrodes. Similarly theuse of a solution of dissimilar liquids where wettability with differentmaterials varies on evaporation as a carrier to selectively locate adissolved material in a structure can be used alone or in connectionwith other processes. Accordingly, this invention is not to be construedas limited by the above disclosure but only by the following claims.

We claim:
 1. A process for introducing a soluble, catalyst containingmaterial into a conducting particle, hydrophobic-bonded electrochemicalcell electrode which comprises:a. dissolving said solublecatalyst-containing material in a liquid solution of dissimilar liquidswhich liquid solution is:i. capable of wetting the hydrophobic bondingmaterial of the electrode at a first composition range of said liquids,ii. incapable of wetting the hydrophobic bonding material at a secondcomposition range of said liquids, and iii. capable of changing fromsaid first composition range to said second composition range onevaporation of said solution; b. contacting said electrode structurewith said soluble catalyst-containing material dissolved in the liquidsolution in said first composition range, whereby said solution entersthe hydrophobic and hydrophilic regions of said electrode structure, andc. evaporating said solution to change its composition from said firstcomposition range to said second composition range whereby said solutionwithdraws from the hydrophobic bonding material of said electrodestructure.
 2. The process of claim 1 further comprising the step ofdepositing insoluble catalyst from solution onto the conductingparticles.
 3. The process of claim 2 wherein said soluble catalystcontaining material is chloroplatinic acid and insoluble platinumcatalyst is deposited by reduction with hot hydrogen.
 4. The process ofclaim 1 wherein said dissimilar liquids are water and an alcohol, saidhydrophobic bonding material is PTFE and said porous conductingparticles are selected from the group consisting of carbon and graphite.